The present invention relates to a process for directly obtaining sulfoacetate derivatives as well as the sulfoacetate derivatives which may be obtained with this process.
The cellulose acetates are the most commonly industrially manufactured cellulose derivatives.
Cellulose acetylation is not actually specific and generally only the cellulose triacetates are directly obtained.
Thus, acetylation rate control is not possible in the course of the reaction, which does involve a major problem as it is generally admitted that the solubility and the properties of the resulting derivative are directly related to the cellulose acetylation degree (number of acetylated hydroxyl functions per anhydroglucose unit).
In most techniques according to the prior art, obtaining hydrosoluble cellulose acetate, usually having an acetylation degree within the range from 0.5 to 1, goes through the formation of a cellulose triacetate followed by hydrolysis of the acetyl groups, for example, according to Tanghe L. J. et al. (1963): xe2x80x9cCellulose acetatexe2x80x9d in Methods in Carbohydrate Chemistry. Vol. 3, Whistler R. L. (ed.) 193-212, Academic Press, New York or according to Samios E. et al. (1997): xe2x80x9cPreparation, characterization and biodegradation studies on cellulose acetates with varying degrees of substitutionxe2x80x9d, Polym., 38 (12): 3045-3054. Said deacetylation may further be performed by methanolysis, for example, according to Buchanan C. M., (1991): xe2x80x9cPreparation and characterization of cellulose monoacetates: the relationship between structure and water solubilityxe2x80x9d, Macromol., 24: 3060-3064.
These prior methods involve a considerable extra costxe2x80x94time, reactive consumptionxe2x80x94which excludes contemplating a commercialisaton of hydrosoluble cellulose acetates. Moreover, during the acetylation and deacetylation steps, a strong depolymerization of these derivatives occurs, which affects their rheological properties.
Indeed, according to Aikhodzhaev B. I., et al. (1982): xe2x80x9cPreparation and study of the properties of primary soluble cellulose acetates with low degree of substitutionxe2x80x9d, it would be possible to prepare hydrosoluble cellulose acetate directly. But this teaching does not mention anything about the production of sulphated derivatives and the described operational conditions seem to lead to a strong depolymerization of cellulose due to the presence of sulphuric acid in a considerable quantity.
The present invention, as far it is concerned, aims at offering a process for directly producing hydrosoluble sulphated cellulose acetate derivatives in that it does not include a preliminary deacetylation step of cellulose triacetates and this, practically, with no depolymerization of the cellulose chain.
More particular, the invention relates to a process for producing hydrosoluble sulphated cellulose acetate derivatives with a polymerization degree equal to at least 0.8 times that of the initial cellulose.
The present invention also relates to cellulose sulfoacetates adapted to be obtained through such a process and which have more particularly the following advantages and/or features:
variable acetylation degrees, depending on the origin of the cellulose of the starting cellulose material,
a sulphation degree ranging from 0.6 to 0.2 (depending on the reaction length), being approximately 0.3 in the generally used reaction conditions,
a controlled polymerization degree,
an excellent solubility in water and polar solvents, and advantageous rheological properties due to their high viscosity, similar to those of associative polymers,
a water retention in the presence of salt: they swell up to 190 ml/g, but remain soluble.
The present invention will now be described more in detail as far as its objects and other features are concerned, and making use of examples given merely by way of illustration and not of limitation, referring to the accompanying drawing in which:
FIG. 1 shows a diagram illustrating an optimization example of the acetic anhydride quantity (in mols of acetic anhydride per mol of anhydroglucose) for an esterification time of 30 minutes, and
FIG. 2 shows a diagram illustrating an optimization example of the cellulose esterification time for an acetic anhydride quantity of 3.2 mols/mol of anhydroglucose.
The present invention more precisely aims at offering a process for directly producing a blend of hydrosoluble cellulose sulfoacetate derivatives through esterification of a cellulose material, characterized in that it involves the steps comprising:
(i) suspending the cellulose material in a glacial acetic acid solution and eliminating the acetic acid excess,
(ii) suspending the cellulose material swollen with acetic acid in a sulphuric acid solution in the glacial acetic acid, and
(iii) reacting the cellulose material by adding the acetic anhydride.
Preferably, the process according to the invention comprises a step (iv) during which the reaction in step (iii) is stopped by adding an aqueous solution of acetic acid.
According to a preferred embodiment, the process according to the invention comprises the steps of:
(v) optionally centrifuging the blend of the above-mentioned step (iv),
(vi) washing and eliminating the optionally obtained culot,
(vii) adding water in order to precipitate the optionally generated cellulose triacetates,
(viii) centrifuging and eliminating the residue,
(ix) neutralizing the supernatant by optionally cooling,
(x) dialyzing the resulting precipitate, and
(xi) freeze-drying the solution.
In such an embodiment, advantageously at step (vi) the optionally obtain residue is washed three times with acetic acid, and then three times with deionized water.
It is also preferable to put, at step (vii), the blend at a temperature within the range from 4 to 25xc2x0 C., preferably about 4xc2x0 C., for a period of time ranging from 1 to 24 hours, preferably about 16 hours.
Still in this embodiment, it is preferred that the step (ix) be performed by a slow addition of a sodium hydroxide solution until a pH of about 7.5 is reached.
Similarly, advantageously this same step (ix) will also be carried out by cooling the blend in an ice bath and to follow the pH continuously so that the pH does not exceed 10, preferably about 7.5.
According to another also preferred embodiment, step (x) of the process according to the present invention may be replaced by several washings with ethanolic solutions advantageously made of 60%, 70%, 80% and 100% ethanol. Step (xi) is then replaced by a drying step of the resulting precipitate, the temperature ranging from about 20xc2x0 C. to about 60xc2x0 C., preferably about 40xc2x0 C.
According to the invention, the method moreover shows the complementary and/or alternative following features:
the chosen acetic anhydride quantity ranges from 3 to 7 mols/mol of anhydroglucose, preferably about 3,2 mols/mol of anhydroglucose,
the chosen esterification time ranges from 1 to 60 minutes, preferably about 20 to 30 minutes,
the chosen esterification temperature ranges from 25xc2x0 C. to about 80%, preferably about 40xc2x0 C.,
the starting cellulose material is selected in the group comprising purified cellulose residues from co-products derived from agriculture and more particularly, from cereal bran, for example wheat and corn, but also from wood cellulose, for example pine-tree . . . , or from commercially-available microcrystalline cellulose of Avicel cellulose type, for example. The process according to the invention thus shows its feasibility on different cellulose sources and thus of various purities, containing notably from 60% to 100% cellulose,
in the case where the cellulose originates from cereal bran, the starting cellulose material is subjected to a preliminary acidic or basic treatment for extracting non cellulose polymers; more preferably, said preliminary treatment is performed in the presence of a reducing agent such as sodium borohydride.
The present invention also aims at a blend of hydrosoluble cellulose sulfoacetate derivatives likely to be obtained with the process according to the invention and characterized in that said derivatives have an acetylation degree ranging preferably from 1.5 to about 2.4. The acetyl groups of the sulfoacetates probably take part in inter-chain interactions able to increase the viscosity in a very large extent.
Preferably the sulfoacetate derivatives according to the present invention have a sulphation degree ranging from about 0.2 to about 0.6, preferably about 0.3.
The sulfoacetate derivatives according to the invention also show the following complementary and/or alternative features and/or advantages:
only the carbon atom which is in position 6 of the anhydroglucose cycles of said derivatives is sulphated; thus, the sulphates increase the cellulose hydrophilic character and tend to make the derivative hydrosoluble,
said derivatives are obtained from cellulose having a viscosimetric mean polymerization degree determined in cupric ethylene diamine at 25xc2x0 C. ranging from 210 to 1 590,
the intrinsic viscosity of the blend of said derivatives, determined by extrapolation at a nil concentration of the reduced viscosity measured in water at 25xc2x0 C. ranges from about 600 to about 1 500 ml/g and comparable to that of carboxymethylcellulose,
said derivatives have water retention properties such that in the presence of salts, they swell up to 200 ml/g but remain insoluble,
the blend of said derivatives is free from triacetylated derivatives which have been eliminated in the course of the process,
said derivatives are thermally stable for 16 hours at 80xc2x0 C., despite the presence of the sulphate groups,
the blend of said derivatives may have the form of a thermoreversible and partially thixotropic gel.